Transform inclusion/composition.

[mbostock]

There should be a way to define a transform and include it in another transform. Perhaps: a.include(b).

[mbostock]

On another note, we should make it more explicit that calling apply on a transform always applies the root transform, rather than the current scope. Or even prevent such mistakes by only defining the apply method on root transforms.

Alternatively, it might be possible to apply a subset of a transform, but I suspect that this will be confusing as the transform will be applied in the global scope, rather than in the context of how it was defined.